3Com S7906E Configuration Manual page 797

S7900e family release 6600 series
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Figure 1-16 Route selection based on MED
As shown in the figure above, Router D learns network 10.0.0.0 from both Router A and Router B.
Because Router B has a smaller router ID, the route learned from it is optimal.
Network
*>i
10.0.0.0
* i
When Router D learns network 10.0.0.0 from Router C which has a smaller router ID than Router B, the
route from Router C becomes optimal, as shown below.
Network
*>i
10.0.0.0
* i
10.0.0.0
* i
However, Router C and Router B reside in the same AS, and therefore BGP will compare the MEDs of
them. Since Router C has a greater MED, network 10.0.0.0 learned from it is not optimal.
In this case, you can configure the bestroute compare-med command on Router D. After that, Router
D will put routes received from the same AS into a group. For the same group, the route with the lowest
MED is selected. Then, it compares routes from different groups. This mechanism avoids the
above-mentioned problem. The following output is the BGP routing table on Router D after the
comparison of MED of routes from each AS is enabled. Network 10.0.0.0 learned from Router C is the
optimal route.
Network
*>i
10.0.0.0
* i
10.0.0.0
* i
Note that, in this case, BGP load balancing cannot be implemented because load balanced routes must
have the same AS-path attribute.
Follow these steps to enable the comparison of MED of routes from each AS:
To do...
Enter system view
Enter BGP view
Enable the comparison of MED of
routes from each AS
NextHop
MED
2.2.2.2
50
3.3.3.3
50
NextHop
MED
1.1.1.1
60
2.2.2.2
50
3.3.3.3
50
NextHop
MED
3.3.3.3
50
2.2.2.2
50
1.1.1.1
60
Use the command...
system-view
bgp as-number
bestroute compare-med
1-28
LocPrf
PrefVal Path/Ogn
0
300e
0
200e
LocPrf
PrefVal Path/Ogn
0
200e
0
300e
0
200e
LocPrf
PrefVal Path/Ogn
0
200e
0
300e
0
200e
Optional
Not enabled by default
Remarks

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